Compressor systems generally include such components as a power driver, a compressor, and a tank for receiving pressurized gases. Typically, electric motors or internal combustion engines are utilized as a power driver in a compressor system. In use, pressurized gas is released from the tank for such purposes as spraying paint or inflating automobile tires. Normally, the compressor operates intermittently according to the demand of the system. When the pressure in the tank drops to a preselected level, the compressor system operates to raise the tank pressure and ceases to operate after the tank pressure reaches a second preselected level.
The operation of the compressor to produce pressurized gases is usually controlled by one of two methods. First, in a drive control mode, an electric motor or an engine driving a compressor is enabled and disabled through a start/stop switch. In the drive control mode, the compressor produces pressurized gases in response to the operation of the motor. Usually, the motor is automatically enabled and disabled in response to detection of certain pressure levels in the tank. Second, in an unloader control mode, an unloader is used to mechanically prevent the compressor from producing pressurized gases even though it is still driven, usually by an electric motor or a gasoline engine. When the compressor is unloaded, the compressor continues to run with only friction losses but no pressure is produced. Usually, the unloader is automatically enabled and disabled in response to detection of certain pressure levels in the tank.
Under light load conditions, a light demand for the pressurized gas, it is preferable to control compressor operation by starting and stopping the electric motor utilizing the drive control mode. Light load conditions are characterized by short run periods for the compressor followed by a relatively long time period in which the system does not require additional pressurized gases. By disabling the motor or engine instead of unloading the compressor mechanically, compressor wear is reduced and no power is consumed during the period of time that the compressor is not required to run.
The unloader control mode is preferable under heavy load conditions where the compressor system in operating near maximum load. Under heavy load conditions, the compressor will be required to start and stop frequently and to run for extended time periods. Since most internal combustion engines and electric motors are very inefficient during start-up, attempts to control the compressor by enabling and disabling the power driver would result in an increased energy requirement due to the frequent start-ups and may possibly lead to engine or motor damage. In contrast, if an unloader is used to control the compressor, the unloaded compressor requires little power input during the relatively short off periods, and the continuous operation of the power driver during the off period generally requires less energy than would a restart of the power driver. Thus unloader control is efficient and economical under heavy load conditions.
Some compressor systems operate under both heavy and light load conditions alternately. In such case, the system is usually provided with two control modes, a drive control mode for enabling and disabling the power driver, and an unloader control mode for mechanically unloading and loading the compressor. Selection between the two control modes is normally accomplished manually. Such manual selection results in inefficient operation of the compressor system because the operator does not quickly respond to changes in demand conditions. Also, manual selection between two control modes is inconvenient and time consuming for the operator. Thus, a need has arisen for an automatic control selector that automatically selects the control mode best suited to control a compressor system according to detected demand conditions.